For this weeks question, the route cause was one of many possibilities that were investigated. The actual issue was identified after validating data used for performing a prepaid balance audit.
A prepaid balance audit is a simple control whereby the following steps are conducted:
Step 1: At a fixed point in time record the total amount of credit held on your prepaid billing system
Step 2: Record the total number of credits(top ups) to the prepay billing system for a given period(say 24hrs) – use an external source like the prepaid card database along with any other crediting mechanisms
Step 3: Record the total amount of debits from prepaid accounts for the same period(as a negative number)
Step 4: at the end of the period record the total credit held on the prepaid billing system
Step 5: Perform the following reconciliation: 1 + 2 + 3 = 4
Now the above didn’t identify the issue, however by validating Step 2 against cash received for sales of top up cards we could see that there was a general issue, where more credits were being applied to the prepaid billing system than those being sold.